1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and more particularly to an apparatus and a method for transmitting pilot signals in a BWA communication system using a plurality of transmit antennas.
2. Description of the Related Art
In a 4th generation (4G) communication system, which is the next generation communication system, research is being performed to provide users with services having various Qualities of Services (QoSs) at a high speed. In particular, in the current 4G communication system, research is being performed to support a high speed service for ensuring mobility and QoS in a BWA communication system, such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system.
Typical types of 4G communication system are an Institute of Electrical and Electronics Engineers (IEEE) 802.16a/d communication system and an IEEE 802.16e communication system. The IEEE 802.16a/d communication system and the IEEE 802.16e communication system utilize an Orthogonal Frequency Division Multiplexing (OFDM) scheme/an Orthogonal Frequency Division Multiple Access (OFDMA) scheme that support a broadband transmission network for a physical channel of the wireless MAN system. The IEEE 802.16a/d communication system and the IEEE 802.16e communication system transmit mass storage data at a high speed using the OFDM/OFDMA scheme. The IEEE 802.16a/d communication system considers only a single cell structure and stationary subscriber stations (SSs), i.e., the system does not accommodate the mobility of the SSs. However, the IEEE 802.16e communication system accommodates the mobility of an SS in the IEEE 802.16a communication system. Herein, an SS having the mobility will be referred to as a Mobile Station (MS).
FIG. 1 is a diagram illustrating a conventional mini sub-channel structure of an IEEE 802.16d communication system using a Single Input Single Output (SISO) scheme. Because the IEEE 802.16d communication system uses the OFDMA scheme, the IEEE 802.16d communication system uses a plurality of sub-carriers and a plurality of sub-channels each of which comprises at least one sub-carrier.
Referring to FIG. 1, a horizontal axis represents a time domain, a vertical axis represents a frequency domain, and one block occupied by the time domain and the frequency domain represents a tone, i.e., a sub-carrier. Herein, a frequency domain occupied by the sub-carrier will be referred to as a ‘sub-frequency domain’. It is noted that the tone is used together with the sub-carrier for convenience of description.
One mini sub-channel 101 comprises a predetermined number of tones, e.g., 18 tones. When the SISO scheme is used, the mini sub-channel 101 comprises a predetermined number of pilot tones, e.g., two pilot tones 102 and 103, for a channel estimation. The remaining tones excluding the pilot tones 102 and 103 represent data tones.
As illustrated in FIG. 1, the pilot tones 102 and 103 are located in central positions of the mini sub-channel 101 for the channel estimation. For example, a transmitter, e.g., a Base Station (BS), transmits the pilot tones 102 and 103, so that a receiver, e.g., an MS or plurality of MSs, can estimate radio channel conditions on the downlink.
Because the mini sub-channel 101 comprises the two pilot tones 102 and 103, a ratio of pilot tones with respect to the total tones is 1/9.
As described above, the MSs estimate radio channel conditions using the pilot tones transmitted from the BS and demodulate received data according to the estimated radio channel conditions. Accordingly, estimating the radio channel conditions has a great influence on the entire system performance.
The sub-channel, which is a basic unit of data transmission in the IEEE 802.16d communication system, comprises three mini sub-channels. Accordingly, it is possible to transmit a symbol including 48 tones through one sub-channel.
The IEEE 802.16d communication system supports a multiple antenna scheme. In the multiple antenna scheme, a BS transmits signals through a plurality of transmit antennas. The multiple antenna scheme may be classified into a Multiple Input Multiple Output (MIMO) scheme and a Multiple Input Single Output (MISO) scheme according to the number of receive antennas used by MSs.
It is generally known that the multiple antenna scheme has number of advantages. For example, the multiple antenna scheme transmits signals through a plurality of transmit antennas, so that the transmitted signals have a plurality of transmit paths. Therefore, it is possible to acquire transmit antenna diversity gain. Further, the multiple antenna scheme transmits signals through a plurality of transmit antennas, so that the transmitted signals have a plurality of transmit spaces. Therefore, it is possible to acquire spatial diversity gain by using a Spatial Multiplexing (SM) scheme.
When the multiple antenna scheme is used as described above, it is possible to acquire the transmit antenna diversity gain and the spatial diversity gain. Accordingly, the multiple antenna scheme is used for efficiently transmitting information data. However, even though the multiple antenna scheme is used, the transmit antenna diversity gain and the spatial diversity gain may be varied according to actual radio channel conditions.
Further, when the multiple antenna scheme is used as described above, the MS must precisely estimate radio channel conditions from each of the transmit antennas to the receive antenna of the MS in order to demodulate the signals transmitted from the BS through each of the transmit antennas because it is possible to acquire the transmit antenna diversity gain and the spatial diversity gain only through the precise estimation of the radio channel conditions. In the conventional wireless communication system, the radio channel conditions are estimated using pilot signals.
However, when the transmit antennas are used as described above, transmit paths experienced by the signals transmitted through each of the transmit antennas may be varied. Therefore, radio channels experienced by the signals transmitted through each of the transmit antennas may also be varied. Accordingly, it is possible to achieve the precise radio channel estimation only when the precise identification of the transmit antennas is possible. In addition, it is possible to precisely demodulate received signals through the precise radio channel estimation. More specifically, because pilot signals are used for the radio channel estimation differently from general information data, the identification of the transmit antennas becomes more and more important.
In order to identify the transmit antennas, the mini sub-channel 101 must transmit pilot signals through each of the transmit antennas at different positions. However, in order to transmit the pilot signals through each of the transmit antennas, it is necessary to reduce the amount of transmittable data. As a result, as the number of the transmit antennas increases, the amount of transmittable data reduces.
For example, when one transmit antenna is used, one mini sub-channel 101 uses only two pilot tones 102 and 103 and can transmit data using the remaining tones, i.e., the data tones, as described in FIG. 1. However, when two transmit antennas, i.e., a first and a second transmit antenna, are used, it is impossible to transmit data through the second transmit antenna in a tone identical to the tone transmitting pilot signals through the first transmit antenna. As described above, the amount of transmittable data is reduced as the number of the transmit antennas increases, thereby decreasing the total system transmission capacity. Consequently, the entire system quality may deteriorate.